This invention relates to an apparatus which detects in a non-contact mode the dielectric constant of a liquid, e.g. a fuel supplied to a combustor or the like, and more particularly to a fuel dielectric constant detecting apparatus for measuring the alcohol percentage content of a fuel used for the engine of an automobile or the like.
Recently, in USA and in many countries in Europe, in order to decrease the consumption of oil and to reduce the contamination of air by automobile exhaust gas, a fuel prepared by mixing alcohol with gasoline has been being introduced for automobiles. If this alcohol-mixed fuel is used, as it is, for the engine the operation of which is matched with the air-fuel ratio of gasoline fuel, then it is difficult to run the engine smoothly because the alcohol is smaller in stoichiometry than the gasoline, and the air-fuel ratio is therefore altered. Hence, in this case, it is necessary to detect the alcohol percentage content of the alcohol-mixed fuel, thereby to adjust the air-fuel ratio and the ignition timing of the engine.
Heretofore, in order to detect the alcohol percentage content, two systems are available. In one of the system, the dielectric constant of an alcohol-mixed fuel is detected. In the other system, the refractive index thereof is detected. The present applicant has filed an application related to the present invention on an apparatus for detecting the dielectric constant of an alcohol-mixed fuel under Japanese Patent Application No. Hei. 3-22488 which corresponds to U.S. Pat. No. 5,225,783.
FIG. 5 shows an example of the apparatus according to the aforementioned Application. In FIG. 5, a sensor section A includes: a cylindrical insulator 1 in the form of a cylindrical container which is made of insulating material such as oil-resisting plastic into which fuel is introduced; an electrically conductive electrode 3 in the form of a cylinder which is provided inside the cylindrical insulator 1 in such a manner that the cylindrical wall of the electrode 3 is coaxial with and substantially in parallel with the cylindrical wall of the insulator 1; a single layer coil 4 wound on the outer cylindrical surface of the insulator 1 in such a manner that it confronts through the cylindrical wall of the insulator 1 with the electrode 3; and lead wires 4a and 4b connected to the coil 4.
Further in FIG. 5, a fuel passageway 2 is defined by the inner cylindrical surface of the cylindrical insulator 1 and the outer cylindrical surface of the electrode 3; a flange 5 to which the electrode 3 is secured is included, and coupled to the cylindrical insulator 1 through a fuel seal 7, thus providing a fuel container (the flange being integral with the electrode 3); and nipples 6 supply fuel to the fuel passageway 2.
As was described above, in the apparatus, the fuel passageway 2 is located outside the electrode 3, and the single layer coil 4 is located outside the fuel passageway 2 through the cylindrical insulator 1. However, it goes without saying that, even if the order of arrangement of those components is reversed, a fuel container equivalent to the above-described one can be obtained. That is, a sensor section A as shown in FIG. 6 falls in the scope of the invention filed under the aforementioned Japanese Patent Application No. 22488/1991. Now, an apparatus shown in FIG. 6 will be described. In FIG. 6, for convenience in description, parts corresponding functionally to those which have been described with reference to FIG. 5 are therefore designated by the same reference numerals or characters, although they may be different in configuration.
The sensor section A, as shown in FIG. 6, comprises: a cylindrical insulator 1 of oil-resisting plastic which can be formed by injection molding; a single layer coil 4 wound on a cylindrical insulating coil bobbin 4c and sealingly included in the cylindrical insulator 1; lead wires 4a and 4b connected to the single layer coil 4; and an electrically conductive electrode 3 which is substantially in the form of a cylinder provided outside the cylindrical insulator 1. The cylindrical wall of the electrode is substantially in parallel with and coaxial with the cylindrical wall of the single layer coil 4. Both end portions of the electrode 3 are coupled through fuel seals 7 to the cylindrical insulator 1, thus forming a fuel container. A fuel passageway 2 is formed between the inner cylindrical surface of the cylindrical electrode 3 and the wall of an annular recess formed in the cylindrical insulator 1. In the wall of the annular recess, the single layer coil 4 is buried in such a manner that it is spaced a predetermined distance from the electrode 3. Nipples 6 are connected to the electrode 3, to lead fuel into the fuel passageway 2.
Further in FIG. 6, a detecting circuit section B comprises: a resistor 10 connected in series to the lead wire 4a of the single layer coil 4, thus forming a series circuit; a 0.degree. phase comparator to which signals provided at both ends of the resistor 10 are applied; a low-pass filter 12 to which the output of the phase comparator 11 is applied; a comparison integrator 13 to which the output of the low-pass filter 12 and a predetermined reference voltage V.sub.ref corresponding to a phase of 0.degree. are applied; a voltage-controlled oscillator 14 to which the output of the comparison integrator 13 is applied; an amplifier 15 for amplifying the output of the voltage-controlled oscillator 14, the output of the amplifier 15 being connected to the aforementioned series circuit; and a frequency divider 16 for dividing the output frequency of the voltage-controlled oscillator 14.
The operation of the conventional apparatus is as follows:
Each of the sensor sections A shown in FIGS. 5 and 6 has an equivalent circuit and characteristics as shown in FIGS. 4 (a),(b) and (c). FIG. 4(a) shows a parallel resonance equivalent circuit; FIG. 4(b) indicates frequencies with sensor section impedances and current-voltage phases; and FIG. 4(c) indicates resonance frequencies with methanol percentage contents.
FIG. 4(a) indicates a current I, a voltage V, an impedance Z, the inductance L of the single layer coil 4, an electrostatic capacitance C.sub.f between the single layer coil 4 and the electrically conductive electrode 3 which changes with the dielectric constant .epsilon. of the fuel in the fuel passageway 2; and a capacitance C.sub.p which is unrelated to the dielectric constant .epsilon. of the fuel such as a stray capacitance of the lead wire 4a or the input capacitance of the phase comparator. When the signal applied to the lead wire 4a of each of the sensors A shown in FIGS. 5 and 6 is changed in frequency, then a parallel LC resonance occurs as shown in FIG. 4b. In this operation, the parallel resonance frequency f.sub.r can be represented by the following Equation (1): ##EQU1##
where K, a and b are the constants determined from the configuration of the sensor section A.
The resonance frequency f.sub.r depends on the dielectric constant .epsilon. of the fuel as is apparent from Equation (1), and it decreases as the dielectric constant .epsilon. of the fuel increases. When measured with the apparatus whose sensor was of a predetermined configuration, the resonance frequency f.sub.r was about 7.5 MHz in the case of a methanol having a dielectric constant .epsilon. of 33, and 9.5 MHz in the case of a gasoline having a dielectric constant .epsilon. of 2. In the cases of mixed fuels prepared by mixing methanol and gasoline optionally, the resonance frequency f.sub.r changed with the methanol percentage content as shown in FIG. 4. Thus, by detecting a signal corresponding to the resonance frequency f.sub.r, the dielectric constant .epsilon. of the fuel, and accordingly the methanol percentage content of the methanol mixed fuel can be detected.
In each of the FIGS. 5 and 6, the detecting circuit section B is so designed as to detect the above-described resonance frequency f.sub.r. Under the condition that the methanol-mixed fuel is flowing in the fuel passageway 2, the voltage-controlled oscillator 14 applies a high frequency signal to the series circuit of the resistor 10 and the single layer coil 4, and high frequency voltage signals at both ends of the resistor 10; that is, a high frequency voltage signal applied to the series circuit and a high frequency voltage signal applied to the single layer coil 4 are applied to the phase comparator 11, where they are subjected to phase comparison. In the case where a high frequency voltage signal having the frequency of which is equal to the resonance frequency f.sub.r is applied to the series circuit, then as shown in the part (b) of FIG. 4 the current-voltage phase of the sensor section A is 0.degree., and therefore the phase shift between the high frequency voltages at both ends of the resistor 10 is 0.degree..
On the other hand, in the case where a high frequency voltage signal whose frequency is lower than the resonance frequency f.sub.r is applied to the series circuit, as shown in FIG. 4(b) the current-voltage phase of the sensor section A is ahead of 0.degree., and therefore the phase shift between the high frequency voltages at both ends of the resistor 10 is larger than 0.degree. with the phase of the high frequency signal as a reference which is applied to the series circuit. Hence, the output of the phase comparator 11 is converted into a DC voltage corresponding to the phase shift with the aid of lowpass filter, and the DC voltage and a reference DC voltage V.sub.ref corresponding to a phase shift of 0.degree. are applied to the comparison integrator 13 where the difference between the DC voltage and the reference DC voltage V.sub.ref is subjected to integration. The output of the comparison integrator 13 is applied to the voltage-controlled oscillator 14 which applies the high frequency signal to the series circuit through the resistor 10, on thus completing, a phase synchronization loop has been formed.
The voltage-controlled oscillator 14 performs a control operation through the phase synchronization loop so that the phase shift between the high frequency voltage signals at both ends of the resistor 10 may be 0.degree.. Hence, the oscillation frequency of the voltage-controlled oscillator 14 is equal to the resonance frequency f.sub.r at all times. And a frequency output f.sub.out is obtained by suitably dividing the output frequency of the voltage-controlled oscillator 14 with the frequency divider 16. Furthermore, since the oscillation frequency of the voltage-controlled oscillator corresponds exactly to the control input voltage, the output of the comparison integrator 13 can be obtained as a voltage output V.sub.out.
However, in the apparatus shown in FIG. 6, it is difficult to reduce the length of the lead wire 4a of the single layer coil 4, although the stray capacity of the lead wire 4a is relatively large. Employment of a shielded wire as the lead wire 4a is effective in eliminating noise signals; however, it will increase the stray capacitance. Even when the shield is separated from the lead wire 4a, the stray capacitance changes depending on the position where the lead wire 4a lies or on the environmental conditions such as for instance ambient humidity.
Such a large stray capacitance means that, in the above-described Equation (1), C.sub.p increases when compared with C.sub.f, and as is apparent from Equation (1), the rate of change of the resonance frequency f.sub.r is decreased with respect to the variation of the dielectric constant .epsilon. of the fuel. When the stray capacitance changes depending on the environmental conditions, the resonance frequency f.sub.r is changed even if the dielectric constant .epsilon. of the fuel is not changed, which adversely affects the accuracy of detection of the dielectric constant of the fuel.